Natural resources such as oil and gas located in a subterranean formation can be recovered by drilling a wellbore down to the subterranean formation, typically while circulating a drilling fluid in the wellbore. After the wellbore is drilled, a string of pipe, e.g., casing, can be run in the wellbore. The drilling fluid is then usually circulated downwardly through the interior of the pipe and upwardly through the annulus between the exterior of the pipe and the walls of the wellbore, although other methodologies are known in the art.
Hydraulic cement compositions are commonly employed in the drilling, completion and repair of oil and gas wells. For example, hydraulic cement compositions are utilized in primary cementing operations whereby strings of pipe such as casing or liners are cemented into wellbores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a wellbore and the exterior surfaces of a pipe string disposed therein. The cement composition is allowed to set in the annular space, thus forming an annular sheath of hardened substantially impermeable cement. This cement sheath physically supports and positions the pipe string relative to the walls of the wellbore and bonds the exterior surfaces of the pipe string to the walls of the wellbore. The cement sheath prevents the unwanted migration of fluids between zones or formations penetrated by the wellbore. Hydraulic cement compositions are also commonly used to plug lost circulation and other undesirable fluid inflow and outflow zones in wells, to plug cracks and holes in pipe strings cemented therein and to accomplish other required remedial well operations. After the cement is placed within the wellbore a period of time is needed for the cement to cure and obtain enough mechanical strength for drilling operations to resume. This down time is often referred to as “waiting-on-cement”, or WOC. If operations are resumed prior to the cement obtaining sufficient mechanical strength, the structural integrity of the cement can be compromised.
Two common pumping methods have been used to place the cement composition in the annulus. The cement composition may be pumped down the inner diameter of the casing and up through the annulus to its desired location. This is referred to as a conventional-circulation direction method. Alternately, the cement composition may be pumped directly down the annulus so as to displace well fluids present in the annulus by pushing them up into the inner diameter of the casing. This is referred to as a reverse-circulation direction method. Cement can also be used within the wellbore in other ways, such as by placing cement within the wellbore at a desired location and lowering a casing string into the cement. The latter method may be used, for example, when there is not the ability to circulate well fluids due to fluid loss into a formation penetrated by the wellbore.
In carrying out primary cementing as well as remedial cementing operations in wellbores, the cement compositions are often subjected to high temperatures, particularly when the cementing is carried out in deep subterranean zones. These high temperatures can shorten the thickening times of the cement compositions, meaning the setting of the cement takes place before the cement is adequately pumped into the annular space. Therefore, the use of set retarding additives in the cement compositions has been required. These additives extend the setting times of the compositions so that adequate pumping time is provided in which to place the cement into the desired location.
While a variety of cement set retarding additives have been developed and utilized, known additives, such as sugars or sugar acids, can produce unpredictable results. Hydroxy carboxylic acids, such as tartaric acid, gluconic acid and glucoheptonic acid are commonly used in oil well cementing as a cement retarder. However, if an excess of hydroxy carboxylic acid is used it can over-retard the set of the cement slurry and thereby causing it to remain fluid for an extended period of time. This over-retardation can result in extended waiting time prior to resuming drilling and may allow gas to invade the slurry thereby causing unwanted gas migration. The extended waiting time results in delays in subsequent drilling or completion activities.
In a number of cementing applications, aqueous salt has been utilized as an additive in cement compositions. The salt, generally sodium chloride, functions as a dispersant in cement slurry, causing the slurry to expand upon setting whereby the attainment of a good bond between the wellbore and casing upon setting of the slurry is enhanced. However, salt saturated slurries can cause problems to bordering formations, and in certain situations salt can be leached out of the cement slurry, which could cause cement failure. Also, certain salts, such as calcium salts, can act as accelerating agents, which reduce the setting time of the cement composition in an attempt to overcome the negative effects of set retarders. However, the presence of a set and strength accelerating agent, such as calcium salt, in the cement composition can increase the risk that the cement composition may thicken or set before placement. Given the complexity of the cement chemistry and the large temperature and pressure gradients present in the wellbore, and the difficulty in predicting the exact downhole temperatures during the placement and setting of the cement, it can be difficult to control the retarding additive and accelerating agent to get the desired setting behavior. Systems generally are over-engineered to have very long setting (or thickening) times in order to ensure that the mix remains fluid until all of the cementitious material is in place which can result in excessive WOC.
Therefore, there is a need for improved set control methods, which bring about predictable cement composition setting times in the subterranean environments encountered in wells. In particular, it is desirable to develop methods for rapidly setting cement-based systems whereby the timing of the setting is under the control of technicians in the field without the risk of premature setting. Thus, a need exists for a method of cementing a wellbore that would simultaneously contain sufficient retarder material to ensure proper pumpability for the desired pumping duration and a sufficient concentration of an accelerator to shorten the setting time, whereby the thickening effect of the accelerator is under the control of technicians in the field.